Novel water-soluble polymeric photosensitizers (SPO) based on starch and containing porphyrin chromophores were synthesized and studied. The polymers were soluble in water and in dimethyl sulfoxide. Photophysical studies and solubilization of molecular probes proved the formation of hydrophobic, rigid microdomains in an aqueous solution of SPO; they were created due to the clustering of porphyrin chromophores attached to the polymer chain. SPO polymers absorbed light from the UV-visible spectral region. The polymers could sensitize photochemical reactions mediated by electron transfer, energy transfer or both, from the singlet-excited state of porphyrin chromophores to the molecules of organic compounds solubilized in the hydrophobic microdomains or residing in the water phase.